More and more motor vehicles are possessing systems for monitoring and/or measuring parameters comprising sensors mounted on said vehicle.
By way of example relating to such systems, mention may be made of monitoring systems comprising sensors fitted to each of the wheels of vehicles, dedicated to measuring parameters, such as pressure and/or temperature of the tires mounted on these wheels, and intended to inform the driver of any abnormal variation in the measured parameter.
These monitoring systems conventionally comprise:                mounted on each of the wheels of the vehicle, an electronic housing incorporating the measurement sensors, a microprocessor, a radio frequency emitter and a button battery,        and, mounted on the vehicle, a central unit for receiving the signals emitted by the electronic housings, equipped with a computer incorporating a radiofrequency receiver connected to an antenna.        
Moreover, usually, these monitoring systems comprise a displacement sensor suitable for being incorporated into each electronic housing and intended to provide information representative of the rotary, rotationally immobile or mobile, state of the wheel, and hence representative of the state of displacement of the vehicle (stationary or rolling).
Such information makes it possible, among other things, to institute two modes of operation of the monitoring systems determining an “awake” state and an “awoken” state of the electronic housings. These two modes of operation are differentiated in particular by the frequency of emission of the signals output by the electronic housings, and are adapted in such a way that this emission frequency is reduced when the electronic housings are in an “awake” state corresponding to the stationary state of the vehicle.
This management of the emission frequency of the signals emitted by the electronic housings leads, in fact, when the vehicle is stationary, to reducing, on the one hand, the consumption of the batteries energizing the onboard microprocessors, and on the other hand, the “pollution” of the environment surrounding the vehicle, and consequently, in particular, the risks of parasitic interference between close vehicles.
The first family of displacement sensors conventionally used for the aforesaid purposes consists of sensors of the accelerometer type whose output signal varies linearly as a function of the speed of rotation of the wheel.
An essential advantage of such sensors lies in the fact that they turn out to lead to low electrical energy consumption, a paramount quality in view of the necessary requirements, in particular regarding lifetime, which the batteries incorporated in the electronic housings must satisfy. Specifically, the response curve of these sensors as a function of the speed of rotation of the wheels being linear, the determination of the rotary state of a wheel entails simply fixing a rolling threshold, and activating the associated sensor instantly for a time of small duration, so as to achieve the comparison of the value of the signal delivered with the rolling threshold.
On the other hand, such sensors exhibit two major drawbacks residing, on the one hand, in their high retail cost, and on the other hand, in their relative fragility in view of the conditions of use.
The second family of displacement sensors conventionally used for the purpose of determining the rotary state of the wheels of vehicles consists of sensors whose output signal exhibits a waveform of variable amplitude, for example sinusoidal, and synchronous with the speed of rotation of the wheel. Such sensors, of the type of impact sensors, magnetic sensors for detecting variations in the terrestrial magnetic field, termed “EMF” sensors, tangential accelerometers, turn out to have a lower retail price and greater robustness relative to those of the sensors of the first family mentioned above.
On the other hand, the current solutions implemented to ensure the processing of the signal delivered by such sensors with a view to determining the rotary state of a wheel turn out to involve significant electrical energy consumption.
The first solution, of the analog type, which consists in using systems of amplifiers and of comparators, makes it necessary, in particular on account of problems of average value, hysteresis, etc., to energize these components for a duration corresponding to the time required for the wheel to perform a revolution at low speed (detection of the starting of the vehicle). Now, such a duration turns out to be very detrimental within the framework of the use of a button battery.
The second solution, of the digital type, which consists in using a conversion system comprising a filter and an analog/digital converter, demands a set-up time equivalent to that of the analog solution, hence likewise very detrimental within the framework of the application to which the invention is directed.